This invention of the continuation-in-part of the aforementioned patent relates generally to the processing of photosensitive lithographic plates, and more particularly to the exposure of photosensitive letterpress plates to light prior to the development of such plates.
Offset lithographic plates are used on offset printing presses where an inked plate comes into contact with a blanket roll which then transfers the image to a sheet of print material, usually newspaper. The exposure of these plates to light is generally carried out automatically by a large exposure machine which typically feed plates one after another (or one pair after another) to an exposure station where the plates are exposed to light and then conveyed out of the machine to a developer. Letterpress lithographic plates, on the other hand, are used on presses where the inked plate is brought into direct contact with the printed material, such as commercial advertising. Letterpress plates are similar to offset plates, except they typically comprise a steel rather than an aluminum substrate, and the photosensitive polymer on the substrate is usually significantly thicker than on an offset plate. Unlike an offset plate, the polymer on a letterpress plate must be "pre-exposed" to activate the polymer prior to the main exposure. For good printing results, the main exposure of a letterpress plate must usually take place within a predetermined period of time (e.g., 30 seconds) following the pre-exposure, which is sometimes referred to in the trade as the "bump".
Prior to the present invention, the pre-exposure and main exposure of a letterpress plate has not been automated. A typical process has involved manually positioning a plate in a pre-exposure machine, pre-exposing the plate to light, and then manually removing the plate from the machine and placing it in a different position for the main exposure. This process is time consuming and expensive.